Boundary layer, a revolutionary approach to simplify the Navier-Stokes equations of fluid flow along a solid surface into two distinctively viscous and inviscid domains, was first proposed in 1904 by Ludwig Prandtl, the father of modern aerodynamics. The boundary layer plays a critical role in determining transport behaviors and hence performances. As such, directly engineering the boundary layer has been recently demonstrated to be effective in achieving unprecedented flight performance. Moreover, during flow boiling in microchannels, the boundary layer governs bubble dynamics (i.e., bubble generation, departure, and interactions) and therefore the two-phase flow and heat transfer behaviors. Thus, favorably manipulating or even controlling the boundary layer might be a promising strategy to drastically enhance flow boiling in microchannels. Unlike the boundary layer in a single-phase flow that can be predicted by Navier-Stokes equations, the boundary layer behaviors in the multi-phase flow are complex and hence extremely challenging to predict and manage.
Two-phase transport in microchannels over the last decade has been extensively studied because of its great importance in microfluidic devices, compact heat exchangers, proton exchange membrane (PEM) fuel cells, and thermal management of high power electronics. In those microsystems with the hydraulic diameter at O (100 μm), the complexity of the boundary layer structure is further exacerbated. Distinct from regular-sized systems, rapidly growing bubbles (as high as 3.5 m/s or 3500 μm/ms) in conventional microchannels will be quickly confined, resulting in Taylor flow in which the liquid is separated by Taylor bubbles (or vapor slugs). With the increase of vapor/gas superficial velocity, the interactions between vapor and liquid flows become more complicated, leading to multiple two-phase flow regimes followed by flow regime transitions. Thus, the development of the boundary layer in two-phase flow is discontinuous and highly dependent on flow regimes. Moreover, during flow boiling in microchannels, Taylor bubbles are subject to a highly non-uniform temperature field, therefore prone to sustaining in a quasi-equilibrium state if not properly managed. These Taylor bubbles can lead to vapor ingestion, flow crisis or even two-phase flow instabilities during flow boiling in conventional microchannels. Equally important, during flow boiling, the confined bubbles can create relatively large dry areas (i.e., the direct contact areas between vapor and walls) on walls with high surface tension fluids such as water and consequently, hinder heat transfer rate and cause premature critical heat flux (CHF) conditions. In addition, if not effectively managed, flow boiling in conventional microchannels is also susceptible with laminar and capillary flow in most typical working conditions with Reynolds number at O (100).
Numerous techniques have been designed to enhance flow boiling in microchannels. These include inlet restrictors (IRs) or orifices to manage reverse flows, seed bubbles to improve thermal equilibrium, impingement and synthetic jets to actively intensify mixing or induce advection, and artificial nucleation cavities such as microfabricated reentry cavities, microcoatings, and nanocoatings to promote nucleate boiling. Most recently, a self-excited and modulated high frequency two-phase oscillation mechanism that allows for passive collapse of confined bubbles inside microchannels has been developed to enhance flow boiling (See e.g., U.S. patent application Ser. No. 13/828,701 of Li, et al. titled “Enhanced Flow Boiling in Microchannels by High Frequency Microbubble-Excited and—Modulated Oscillations” and published as U.S. Publication No. 2014/0027005, which is incorporated by reference herein). The concept to enhance flow boiling heat transfer by separating two phase flows in regular-sized channels and microchannels has been eleganetly achieved. Textured superhydrophobic boiling surfaces was also reported to effectively manipulate nucleate boiling by suppressing film boiling.
Nonetheless, multiphase transport in microchannels remains essential for a wide range of emerging technologies such as microfluidics, direct cooling of high power electronics, and water management in fuel cells. Despite extensive progress over the past decade, it remains challenging to achieve exceptional flow boiling enhancements in microchannels due to unfavorable size effects such as bubble confinement and exacerbated flow instabilities. Since the performances of multiphase transport are intrinsically governed by the boundary layer, the ability to favorably manipulate or even control the boundary layer is strongly desired. Radically different from the single-phase flow in conventional microchannels in which the boundary layer is typically laminar, however, the boundary layer behaviors in the multiphase flow are highly stochastic and transitional, thus extremely difficult to predict and manage.
Furthermore, interest in two-phase transport in microfluidic systems has been rapidly growing because of its wide range of applications in diverse scientific and engineering disciplines including biology, chemistry, and thermal management. For an example, the continuous advances in integrated circuits (ICs) technology has led to unprecedented cooling needs with heat fluxes ranging from approximately 100 W/cm2 in current electronic microchips to 2000 W/cm2 in semiconductor lasers. Dissipating such high heat fluxes with requirements in the temperature uniformity and integration (or compactness) has imposed practical limits on traditional air and single-phase liquid cooling technologies. With the potential to be embedded in microchips, heat transfer in microchannels, has been an active research area ever since the ground-breaking work of Tuckerman and Pease in 1981, which demonstrated the potential of microchannels to dissipate high heat fluxes.
Compared to single-phase transport in microchannels, flow boiling has several key beneficial characteristics. These characteristics include improved temperature uniformity, i.e., lower temperature difference between inlet and outlet, and reduced pumping power due to the latent heat evaporation. The classic two-phase flow patterns, primarily including bubbly flow, slug flow, churn flow and annular flow, carry some unique traits at the micro scale. Because two-phase flow pattern transitions in conventional microchannels are challenging to predict, often transport processes at the micro scale are not designed properly, which in turn, hinders performance and can cause severe two-phase flow instabilities.
More particularly, flow boiling in miniaturized channels has been extensively studied in the last decade. Tremendous progresses have been made in understanding transport mechanisms in heat transfer, two-phase flow instabilities, and critical heat flux (CHF). The prior related work can be classified on several axes, as described below.
In small scale channels, the confinement of the bubble introduces one type of noticeable instabilities termed the rapid bubble growth. This often reported rapid growth of bubbles in the bubbly flow regime in microchannel systems is characterized by high departure frequencies on the order of f=O (10-1000 Hz). In the initial stage of the nucleation cycle, a spherical bubble grows until it attains a size comparable to the channel hydraulic diameter. The bubble then grows rapidly in the longitudinal direction (downstream as well as upstream) causing flow reversal. This, in turn, introduces appreciable disturbances to the flow, and in many cases prematurely triggers other instability modes, such as Ledinegg instability, upstream compressible flow instability, and CHF conditions.
Extensive studies have been conducted in two-phase flow instabilities in microchannels in the last decades. These methods include modifying IRs, improving nucleate boiling, reducing the influence of the surface tensions, creating diverging channel cross-section configurations, and applying micro-jets. Most recently, microfluidic transistors have been developed to enable a self-sustained high frequency two-phase oscillation mechanism and successfully applied it to enhance flow boiling in microchannels. To date, IRs has been found to be the most effective way to suppress Ledinegg instability. However, they introduce dramatic increase in the pressure drop. Additionally, low heat transfer rate results from adding surfactants, and challenges persist in arranging micro-nozzles and compressors when using micro-jets.
In the latest critical reviews on flow boiling in microchannels, it has been demonstrated that flow boiling HTC curves for microchannels are “M” shaped or “U” shaped when varying with thermodynamic equilibrium quality. The downturn of the HTC curve is caused by the confined bubbles (or vapor slugs), where thin film evaporation occurs near the small liquid bridge area; while large wall area within the confined bubbles are devoid of liquid.
It is more challenging to enhance convective flow boiling in microchannels. This is because most operating conditions are laminar. Micro jet arrays have been demonstrated to effectively enhance the boiling process, but the packaging of the impingement jet is still challenging due to the jets arrangement, the flow distribution management and availability of a proper compressor. To enhance nucleate boiling and improve thin film evaporation is another strategy to enhance flow boiling by integrating artificial nucleation cavities and nanowires into microchannels. These include microfabricated reentry cavities, microcoatings, nanocoatings, etc. To date, two-phase flows in miniaturized channels are still limited by bubble confinements, laminar and capillary flows, which result in unpredictable flow pattern transitions and tend to induce severe two-phase flow instabilities and suppress evaporation and convection. This, in turn, is detrimental to heat transfer. As a result, two-phase cooling has not been accepted as a practical approach for electronics cooling.
Compared to single-phase cooling in microchannels, through the latent heat evaporation, flow boiling has great potentials in achieving high temperature uniformity (i.e., low temperature difference between inlet and outlet) at a high working heat flux with a reduced pumping power. Recent studies demonstrated that novel configurations, such as microfluidic transistors, inlet restrictors (IRs) or valves/orifices, artificial cavities, and impingement jets, can suppress boiling instabilities and enhance several key flow boiling parameters including onset of nucleate boiling (ONB), heat transfer coefficient (HTC), and critical heat flux (CHF) conditions. However, flow boiling in miniaturized channels is hampered by several severe constraints such as bubble confinements, viscosity and surface tension force-dominated flows, which result in unpredictable flow pattern transitions and tend to induce severe two-phase flow instabilities and suppress evaporation and convection. This, in turn, is detrimental to flow boiling heat transfer.
As stated, heat and mass transfer are ultimately governed by boundary layers (BLs) during flow boiling in microchannels. It was experimentally demonstrated in recent studies that flow boiling can be enhanced by disturbing BLs through creating oscillations, introducing capillary flows along walls, and promoting thin film evaporation.
However, research to enhance flow boiling in microchannels by intentionally constructing and optimizing BLs has not been reported.